drm/amdgpu: drop amdgpu_atombios_scratch_regs_save/restore

No longer used.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>